drm/doc: Remove outdated FIXME for the page_flip callback

Since the drm_event cleanup work (as prep for fence support) drivers
don't need to bother themselves any more with this, the drm event core
takes care of that.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-8-git-send-email-daniel.vetter@ffwll.ch